Laser Machines and Processing Systems

 

1. Principles of Laser Generation

1.1 Principle of Laser Generation

The atomic structure resembles a miniature solar system, with the nucleus at the center and electrons continuously orbiting around it, while the nucleus also rotates. The nucleus consists of protons and neutrons; protons are positively charged, and neutrons are neutral. The total positive charge of the nucleus is balanced by the total negative charge of the electrons, so atoms are generally electrically neutral.

In terms of mass, the nucleus contains most of the atom's mass, while the mass of the electrons is relatively small. The nucleus occupies a very small space compared to the much larger volume occupied by the electrons. Atoms have "internal energy," which consists of two parts: the kinetic energy from the electrons' orbital motion and the potential energy due to the distance between the negatively charged electrons and the positively charged nucleus. The total energy of an atom, known as its internal energy, is the sum of the kinetic and potential energies of all electrons.

Electrons orbit the nucleus, with those closer to the nucleus having lower energy and those further away having higher energy. The probability of electrons being at different distances leads to the division of electron layers into different "energy levels." Within a given energy level, multiple electrons may occupy similar energy states, but they do not have fixed orbits. The concept of energy levels divides electrons by energy and orbital space into various layers. Thus, an atom can have multiple energy levels, each corresponding to different energy states, with some electrons in lower energy states and others in higher ones.

Physics textbooks now clearly outline the structural characteristics of certain atoms, the distribution of electrons in various layers, and the number of electrons at different energy levels.

In an atomic system, electrons generally move in layers, with some atoms having electrons in higher energy levels and others in lower ones. Because atoms are influenced by external factors such as temperature, electricity, and magnetism, high-energy electrons are unstable and may spontaneously transition to lower energy levels. This transition can lead to "spontaneous emission" or "stimulated emission" of radiation.

• Spontaneous Emission: High-energy electrons, influenced by external conditions, may spontaneously drop to lower energy levels and emit excess energy as photons. This emission is random and incoherent, with each electron's transition being independent and the emitted photons having different states and directions. Spontaneous emission is characterized by its incoherent nature and scattered direction. However, it carries the intrinsic characteristics of the atom, and different atoms have different emission spectra. This relates to the fundamental concept in physics that "any object has thermal radiation ability, with electromagnetic waves being absorbed and emitted continuously. The spectrum of emitted thermal radiation depends on the object's characteristics and temperature." Thus, thermal radiation is a result of atomic spontaneous emission.

Stimulated Emission: high-energy electrons, when exposed to "suitable photons" or "stimuli," transition to a lower energy level and emit a photon with the same frequency as the incident photon. The key feature of stimulated emission is that the emitted photons are identical to the incident photons in terms of frequency, direction, and phase. This results in "coherent" light, where the emitted photons are indistinguishable from the incident ones.

In other words, stimulated emission effectively doubles the number of photons, as one photon is converted into two identical photons. This process amplifies the light, meaning that light is "amplified" or "intensified."

 

To obtain more and more frequent stimulated emission, what conditions are required?

Under normal circumstances, the number of electrons in high energy levels is always less than the number of electrons in low energy levels. To make an atom produce stimulated emission, we need to increase the number of electrons in high energy levels. Therefore, a “pumping source” is needed to excite more electrons from low energy levels to high energy levels. This way, the number of electrons in high energy levels will exceed those in low energy levels, resulting in “population inversion.” The excess electrons in high energy levels will transition to low energy levels in a very short time, increasing the likelihood of stimulated emission.

Of course, the “pumping source” is set according to different atoms. It should make the electrons “resonate” and encourage more electrons in low energy levels to transition to high energy levels.

Readers can generally understand what a laser is and how it is produced. A laser is “light radiation” emitted from atoms in a material under the action of a specific “pumping source.” This is how a laser is generated.

 

1.2 Conditions for Laser Generation

1.2.1 Stimulated Emission Light Amplification

The stimulated emission process can achieve a geometric increase in the number of photons with the same state (frequency, phase, vibration direction, and propagation direction), causing light amplification. An external photon triggers a large number of excited particles to produce stimulated emission and generate a large number of photons with completely identical motion states. This phenomenon is called stimulated emission light amplification.

 

However, when light interacts with an atomic system, there are always three simultaneous processes: spontaneous emission, stimulated emission, and stimulated absorption. Whether light is amplified depends on which transition process dominates. To achieve laser action, stimulated emission must be dominant, which means solving two fundamental contradictions: the contradiction between stimulated emission and stimulated absorption, and the contradiction between stimulated emission and spontaneous emission.

1.2.2 Energy Level System of Active Particles

To form a stable laser, there must be luminescent particles that can create population inversion, known as active particles. These can be molecules, atoms, or ions. Some active particles can exist independently, while others must be embedded in certain materials that provide a host environment. The material that provides the hosting environment for active particles is called the matrix. The matrix and active particles together are referred to as the laser working substance.

Not all materials can achieve population inversion, and even among those that can, not all can achieve population inversion between any two energy levels. If the excited state lifetime of the pumped particles is very short and they cannot accumulate in large numbers within a certain time, population inversion cannot be achieved. Therefore, the working substance needs to have a metastable state structure, i.e., a suitable energy level system.

The energy level system must include both the upper laser level and the lower laser level. In addition, it often requires other energy levels related to laser generation. Typically, laser working substances are composed of atomic systems with three-level or four-level structures containing metastable states.

 

1.3 Special Properties of Lasers

1.3.1 Directional Emission

Ordinary light sources emit light in all directions. To make ordinary light travel in a specific direction, optical devices such as reflectors are used, like those in car headlights and searchlights.

Laser light, on the other hand, is emitted in a single direction by nature, with a very small divergence, approximately 0.001 radians, making it nearly parallel. In 1962, humanity first used a laser to beam at the moon, which is about 380,000 kilometers from Earth. However, the spot of light on the moon's surface was less than two kilometers in diameter. If a searchlight with excellent focusing were used to illuminate the moon, its light spot would cover the entire moon.

1.3.2 Extremely High Brightness

Before the invention of the laser, high-pressure xenon lamps were the brightest artificial light sources, comparable in brightness to the sun. However, the brightness of ruby lasers can exceed that of xenon lamps by several billion times.

Lasers have extremely high brightness and can illuminate objects at great distances. For instance, the beam from a ruby laser on the moon can produce an illuminance of 0.02 units, with a bright red color and a clearly visible spot. In contrast, the brightest searchlight would produce an illuminance of only about one trillionth of a unit on the moon, which is imperceptible to the human eye. The extremely high brightness of lasers is primarily due to their directional emission. A large number of photons are concentrated in a very small spatial area, resulting in high energy density.

1.3.3 Pure Color

The color of light is determined by its wavelength (or frequency). Light of a specific wavelength corresponds to a specific color. The wavelength of sunlight ranges from about 0.76 micrometers to 0.4 micrometers, corresponding to seven colors from red to violet, so sunlight is not monochromatic.

Common monochromatic light sources, such as krypton lamps, helium lamps, neon lamps, and hydrogen lamps, emit only one color of light. Although monochromatic light sources have a single color, they still have a certain range of distribution. For example, a krypton lamp emits red light, but it contains dozens of shades of red.

Basic physics tells us that the narrower the wavelength distribution range, the better the monochromaticity. The wavelength distribution range of laser output is extremely narrow, so the color is extremely pure. For example, the wavelength distribution range of a helium-neon laser outputting red light can be as narrow as nanometers, which is one two-hundredth of the wavelength distribution range of red light emitted by a krypton lamp. Thus, the monochromaticity of lasers far exceeds that of any monochromatic light source.

1.3.4 Good Coherence

Physics tells us that the conditions for two beams of light to produce interference are: the same frequency, the same vibration direction, and a constant phase difference.

The light emitted by lasers has consistent frequency, vibration direction, and phase; when two laser beams overlap in space, the light intensity distribution in the overlap area shows stable interference patterns. Therefore, lasers produce coherent light. In contrast, light from ordinary light sources has varying frequencies, vibration directions, and phases, making it incoherent light.

1.3.5 Extremely Short Pulse Duration

Ordinary light sources cannot produce very short pulses. For instance, the flash duration of a photographic flash is about one thousandth of a second. Pulse lasers, however, can have extremely short pulse durations, down to 6 femtoseconds (1 femtosecond = $10^{-15}$ seconds).

1.3.6 Laser Frequency

Different lasers emit light at different power levels, but their frequencies fall between infrared and ultraviolet.

 

1.4 Lasers

The basic components of a laser are the pumping source, the resonator cavity, and the laser working medium. A detailed introduction to each component will be provided in subsequent sections.

 

 

 

Pumping Source: This is used to excite the laser working medium and elevate the activating particles from the ground state to a higher energy level to achieve population inversion. Pumping sources can include optical excitation, gas discharge excitation, chemical excitation, nuclear excitation, etc. The choice of the pumping source depends on the characteristics of the working medium. Different working media often require different pumping sources. For example, solid-state lasers typically use optical excitation methods like pulsed xenon lamps or tungsten lamps, while gas lasers use electrical excitation through discharge to directly excite the working medium. Additionally, the choice of pumping source should consider factors such as excitation efficiency.

Resonator Cavity: This component increases the effective length of the working medium, selects the directionality of the beam, and determines the laser frequency. The optical resonator cavity is the most crucial factor in determining the laser's output characteristics such as monochromaticity, directionality, and coherence. More detailed information will be discussed in later sections.

Working Medium: This is the material system that achieves population inversion and produces the stimulated emission amplification. Laser media can be gases, liquids, solids, or semiconductors, and they must have metastable states. To select a suitable laser working medium, a spectral analysis of the material is required. Based on this, different materials are chosen according to specific needs. This is a complex issue, involving many factors, the most important of which is whether a material has suitable transition levels that can achieve population inversion between two energy levels.

Here are brief descriptions of several types of lasers:

Ruby Laser: The ruby laser was the first to achieve laser action and emits red laser light at 694.3 nm. Its working medium is a ruby rod made of chromium-doped aluminum oxide, and it operates through stimulated emission of trivalent chromium.

YAG Laser: The YAG laser is currently one of the best-performing solid-state lasers in the mid-to-low power range and can emit light at several wavelengths. Its working medium is a neodymium-doped garnet rod, and it operates through stimulated emission of trivalent neodymium.

Helium-Neon Laser: The helium-neon laser is one of the earliest gas lasers and can emit laser light at wavelengths of 632.8 nm, 1150 nm, and 3390 nm. It has good monochromaticity, a simple structure, and stable output power, making it widely used. The laser lines are produced between different excited states of neon atoms, with helium atoms playing a supportive role. Although the energy level structure of neon is complex, it can be viewed as a four-level system.

2.1 Disk Laser

The disk laser is a diode-pumped solid-state laser, first demonstrated in the early 1990s by Adolf Giesen at the University of Stuttgart. The gain medium in the thin disk is a crystal, usually Yb, Nd, or a ytterbium-doped gain medium for broad wavelength tuning.

Currently, disk lasers are represented by products from Trumpf. The design concept of disk lasers effectively solves the thermal effects problem of solid-state lasers and achieves a perfect combination of high average power, high peak power, high efficiency, and high beam quality. Disk lasers have become indispensable new processing laser sources in fields such as automotive, shipbuilding, railways, aviation, and energy. Trumpf is currently the only company worldwide with the technology to produce high-power disk lasers, with the highest power reaching 16 kilowatts and beam quality achieving 8 milliradians. This has enabled remote laser welding with robotic arms and high-speed large-area laser cutting, opening up a broad application market for solid-state lasers in high-power laser processing.

Advantages of Disk Lasers:

1. Modular Structure:

   Disk lasers adopt a modular structure, allowing for quick replacement of individual modules on-site. The cooling and light-guiding systems are integrated with the laser source, making the structure compact, space-saving, and quick to install and adjust.

2. Excellent Beam Quality and Standardization:

   All disk lasers from Trumpf with power greater than 2 kW have standardized beam parameter products (BPP) of 8 mm/mrad. The laser's performance does not vary with different operating modes and is compatible with all optical components from Trumpf.

3. Lower Optical Power Density:

   Due to the large spot size inside the disk laser, the optical power density on each optical component is lower. The damage threshold for optical coatings is usually about 500 MW/cm², and for quartz, it is 2-3 GW/cm². The power density inside Trumpf disk laser resonators is typically less than 0.5 MW/cm², and on the coupling fibers, it is less than 30 MW/cm². Such low power densities prevent damage to optical components and non-linear effects, ensuring operational reliability.

4. Real-time Power Feedback Control System:

   The real-time feedback control system maintains stable power delivery to the workpiece, providing excellent repeatability in processing results. Disk lasers have nearly zero warm-up time, with a power adjustment range from 1% to 100%. The system thoroughly addresses thermal lensing effects, ensuring stability in laser power, spot size, and beam divergence across the entire power range, without distortion of the beam wavefront.

5. Plug-and-Play Fiber Replacement:

   In the event of a fiber fault, the fiber can be replaced without shutting down the laser. Other fibers can continue to output laser light. Fiber replacement is simple, plug-and-play, requiring no tools or alignment adjustments. The fiber connectors have dust protection to prevent dust from entering the optical component area.

6. Safety and Reliability:

   During processing, even if the material being processed has high emissivity causing laser reflection back into the laser, it has no effect on the laser itself or the processing results. There are no restrictions on material processing or fiber length. Laser operation safety is certified by German safety standards.

7. Simplified Pump Diode Module Replacement:

   The pump diode array module, which is also modular, has a long service life, with a warranty period of 3 years or 20,000 hours. Modules can be replaced without shutdown, whether for planned maintenance or unexpected failures. In the event of a module failure, the control system will alarm and automatically increase the current to other modules to maintain constant laser output power, allowing continued operation for hours or even days. Replacing pump diode modules on-site is straightforward and does not require operator training.

 

2.2 Fiber Laser

Fiber lasers are represented by IPG.

Principle: Similar to other lasers, fiber lasers consist of three main components:

1. Gain Medium (Doped Fiber): This medium generates photons.
2. Optical Resonator Cavity: Provides feedback for photons and performs resonant amplification within the gain medium.
3. Pumping Source: Excites photon transitions.

The principle is illustrated as shown in the figure below.

 

 

The pump light emitted by the pump source is coupled into the gain medium through a reflector. Since the gain medium is a rare-earth-doped fiber, the pump light is absorbed, and the rare-earth ions, having absorbed photon energy, undergo energy level transitions to achieve population inversion. The inverted particles then pass through the resonator, transitioning from the excited state to the ground state, releasing energy and forming a stable laser output.

Features:

1. The fiber has a high “surface area/volume” ratio, which provides excellent heat dissipation and allows for continuous operation without requiring additional cooling.
2. As a waveguide medium, the fiber has a small core diameter, allowing high power density to form within the fiber. Therefore, fiber lasers have high conversion efficiency, low threshold, high gain, narrow linewidth, and minimal coupling losses.
3. Due to the excellent flexibility of the fiber, fiber lasers are compact, versatile, cost-effective, and easy to integrate into systems.
4. The fiber also has many adjustable parameters and selectivity, enabling a wide tuning range, good dispersion properties, and stability.

Classification of Fiber Lasers:

1. Rare-earth-doped fiber lasers
2. Commonly used rare-earth elements in active fibers include Erbium, Neodymium, Praseodymium, Thulium, and Ybrium.
3. Fiber Raman scattering lasers

Summary: Fiber lasers essentially function as wavelength converters, converting the pump wavelength into a specific wavelength of light and outputting it in laser form. From a physics perspective, the principle of light amplification is to provide the working material with light of a wavelength it can absorb, thereby activating the material. Depending on the doping materials, the corresponding absorption wavelengths vary, thus requiring different pump wavelengths. For example, Erbium-doped fiber lasers use pump wavelengths of 800 nm, 980 nm, or 1480 nm to produce laser wavelengths of 1550 nm. Neodymium-doped fiber lasers use pump wavelengths of 800 nm, 980 nm, or 530 nm to produce laser wavelengths of 900 nm, 1060 nm, or 1350 nm.

 

2.3 Semiconductor Lasers

Semiconductor lasers were successfully excited in 1962 and achieved continuous output at room temperature in 1970. Later, improvements led to the development of double-heterojunction lasers and stripe-structured laser diodes, which are widely used in optical fiber communication, optical discs, laser printers, laser scanners, and laser pointers. They are currently the most produced type of laser.

Advantages of laser diodes include: high efficiency, small size, light weight, and low cost. Especially, multiple quantum well types have efficiencies of 20-40%, while P-N types reach around 15%-25%. Overall, high energy efficiency is their greatest feature. Additionally, they provide continuous output wavelengths covering infrared to visible light, and products with optical pulse outputs reaching 50W (pulse width 100ns) are commercially available, making them easy-to-use examples of lasers for applications such as lidar or excitation sources.

According to the solid-state band theory, in semiconductor materials, the energy levels of electrons form bands. The higher energy band is the conduction band, and the lower energy band is the valence band, separated by a forbidden band. When non-equilibrium electron-hole pairs in the semiconductor recombine, the released energy is radiated as light, which is called carrier recombination luminescence.

The commonly used semiconductor materials fall into two categories: direct bandgap materials and indirect bandgap materials. Direct bandgap semiconductors, such as GaAs (gallium arsenide), have much higher radiative transition probabilities and much higher light-emitting efficiency compared to indirect bandgap semiconductors like Si.

Semiconductor lasers operate by injecting carriers and require three basic conditions to emit laser light:

1. To achieve a sufficient population inversion, i.e., having a high enough number of particles in the high-energy state compared to those in the low-energy state.
2. A suitable resonator must be present to provide feedback, causing stimulated emission of photons to amplify and generate laser oscillation.
3. Certain threshold conditions must be met to ensure that photon gain equals or exceeds photon loss.

The working principle of semiconductor lasers involves using the excitation method to create light emission from transitions between energy bands in semiconductor materials (i.e., electrons). Parallel reflective surfaces are formed using the cleaved planes of the semiconductor crystal to create a resonator, causing optical oscillation, feedback, and amplification to produce laser light.

Advantages of semiconductor lasers include small size, light weight, reliable operation, low power consumption, and high efficiency.

Development Process: Low-power LDs used in information technology have developed rapidly. For example, distributed feedback (DFB) and dynamic single-mode LDs for optical fiber communication and optical switching systems, narrow linewidth tunable DFB-LDs, visible wavelength LDs for optical disc information processing (e.g., red light at 670 nm, 650 nm, 630 nm to blue-green light), quantum well surface-emitting lasers, and ultrafast pulse LDs have all seen significant advancements. These devices are characterized by: single-frequency narrow linewidth, high-speed rates, tunability, shorter wavelengths, and optoelectronic integration.

In 1983, a single LD with a wavelength of 800 nm had an output power exceeding 100 mW. By 1989, an LD with a 0.1 mm stripe width reached 3.7 W continuous output, while a 1 cm line array LD achieved 76 W output with a conversion efficiency of 39%. In 1992, the benchmark was raised: a 1 cm line array LD reached a continuous wave output power of 121 W with a conversion efficiency of 45%. High-power LDs with outputs of 120 W, 1500 W, 3 kW, etc., have also emerged. The rapid development of high-efficiency, high-power LDs and their arrays has strongly supported the rapid development of solid-state lasers pumped by semiconductor lasers (LDP).

To meet the needs of EDFAs and EDLs, high-power LDs at 980 nm have also seen significant development. Combined with fiber Bragg gratings for wavelength filtering, this has greatly improved output stability and pump efficiency.

 

2.4 YAG Lasers

YAG lasers are a type of laser. YAG stands for Yttrium Aluminum Garnet (Y₃Al₅O₁₂), a laser medium with excellent overall performance (optical, mechanical, and thermal). Like other solid-state lasers, a YAG laser essentially consists of a laser working substance, a pump source, and a resonator.

However, YAG lasers can be further categorized based on different factors, such as the type of activator ions doped into the crystal, the pump source and pumping method, the structure of the resonator, and other functional components. For example, YAG lasers can be divided into various types based on output waveform (continuous wave YAG lasers, repetitive frequency YAG lasers, and pulsed lasers), working wavelength (1.06 μm YAG lasers, frequency-doubled YAG lasers, Raman-shifted YAG lasers, and tunable YAG lasers), doping elements (Nd

lasers, and YAG lasers doped with Ho, Tm, Er, etc.), crystal shape (rod and slab YAG lasers), and output power (high power and medium-low power YAG lasers).

 

In a solid YAG laser cutting machine, a 1064 nm wavelength pulsed laser beam is expanded, reflected, and focused to irradiate and heat the material's surface. The surface heat then diffuses inward through thermal conduction. By digitally and precisely controlling parameters such as laser pulse width, energy, peak power, and repetition frequency, the material is instantaneously melted, vaporized, and evaporated. This enables cutting, welding, and drilling along predefined trajectories via a CNC system.

Characteristics: The machine features excellent beam quality, high efficiency, low cost, stability, safety, precision, and reliability. It integrates multiple functions such as cutting, welding, and drilling, making it an ideal precision, high-efficiency, and flexible processing device. It offers fast processing speed, high efficiency, good economic benefits, minimal kerf width, smooth cutting surfaces, large depth-to-width ratios, minimal thermal deformation, and can process various materials such as hard, brittle, and soft materials. There are no issues with tool wear or replacement, and it is easy to automate with minimal mechanical changes. The pump efficiency is high, reaching around 20%, and with increased efficiency, the thermal load on the laser medium decreases, significantly improving beam quality. It also has a long lifespan, high reliability, compact size, and lightweight, suitable for miniaturized applications.

Applications: YAG lasers are suitable for laser cutting, welding, and drilling of metal materials, such as carbon steel, stainless steel, alloy steel, aluminum and alloys, copper and alloys, titanium and alloys, and nickel-molybdenum alloys. They are widely used in industries such as aerospace, weaponry, shipbuilding, petrochemical, medical, instrumentation, microelectronics, and automotive. They not only improve processing quality but also enhance work efficiency. Additionally, YAG lasers can provide a precise and fast research tool for scientific research.

Compared to other lasers:

1. YAG lasers can operate in both pulsed and continuous modes. Pulsed output can be obtained using Q-switching and mode-locking techniques to achieve short and ultrashort pulses, offering a wider processing range than CO2 lasers.
2. They have an output wavelength of 1.06 μm, which is an order of magnitude smaller than the 10.6 μm wavelength of CO2 lasers, resulting in higher coupling efficiency with metals and better processing performance.
3. YAG lasers have a compact structure, light weight, ease of use, reliability, and lower maintenance requirements.
4. YAG lasers can be coupled with optical fibers, allowing for time and power multiplexing systems to easily transmit a single laser beam to multiple stations or remote locations, facilitating flexible laser processing.

Therefore, when selecting a laser, various parameters and actual needs must be considered to ensure the laser performs at its maximum efficiency.

Xinte Optoelectronics provides reliable and stable pulsed Nd

lasers for industrial and scientific applications, delivering up to 1.5 J of pulsed output at 1064 nm with a repetition frequency of up to 100 Hz.

  

3. Laser Parameters

3.1 Understanding the M² Factor of Lasers

For laser beams, the beam propagation factor M² is commonly used to characterize the beam quality. The M² factor compares the actual shape of the beam to that of an ideal Gaussian beam.

In the formula, $\omega$ is the beam waist radius of the actual beam, $\theta$ is the divergence half-angle of the actual beam; $\omega_0$ is the beam waist radius of the ideal beam, and $\theta_0$ is the divergence half-angle of the ideal beam. Essentially, M² determines the degree to which the beam matches the perfect Gaussian. The divergence angle $\theta_{R-real}$ of a real laser beam will be greater than the divergence angle $\theta_{G-Gaussian}$ of the ideal Gaussian beam. The wavefront of the beam is flat near the beam waist in the near field and curved in the far field, as shown in the figure below.

ISO standard 11146 defines the M² factor as:

 

There are many quantitative definitions of the divergence angle. The most commonly used definition is that the beam divergence angle is the derivative of the beam radius with respect to the far-field axial position, i.e., when the distance from the waist is much greater than the Rayleigh length. This definition leads to the concept of the half-angle divergence (measured in radians), which depends on the definition of the beam radius. For Gaussian beams, the beam radius is often defined and discussed in detail under the beam radius entry. Sometimes the full angle is used, which is twice the half-angle divergence. In addition to using the angle corresponding to the peak intensity for the divergence angle in Gaussian beams, the full-width at half-maximum (FWHM) divergence angle can also be used. This is commonly employed in data sheets for laser diodes and light-emitting diodes. In Gaussian beams, the divergence angle defined this way is 1.18 times the half-angle divergence determined by the Gaussian beam radius.

Given the beam radius, a larger beam divergence angle, i.e., a larger beam parameter product, is related to the beam quality and represents a lower likelihood of converging the beam into a very small spot. If the beam quality is characterized by a factor M, then the half-angle divergence is:

For example, a Nd laser producing a 1064 nm beam with ideal beam quality has a beam radius of 1 mm and a half-angle divergence of only 0.34 mrad = 0.019°.

To measure the beam divergence angle, it is common to measure the beam's focusing characteristics, i.e., using a beam analyzer to measure the beam radius at different positions.

The beam divergence angle can also be obtained from the complex amplitude distribution at a given plane. This data can be obtained using a Shack-Hartmann wavefront sensor.

3.3 Gaussian Beams

Laser beams produced by lasers are neither plane waves nor uniform spherical waves. Although at a specific location it may appear as a spherical wave, its amplitude and wavefront are constantly changing. Theoretically, after an infinite number of reflections in a stable laser resonator, the laser emitted will propagate in space in the form of a Gaussian beam. Moreover, the more reflections (diffractions) occur, the closer the beam propagation shape will be to a Gaussian beam. Conversely, the closer the shape is to a Gaussian beam, the less it changes in shape during propagation, coupling, and beam transformation. In the case of a Gaussian beam, no matter how it is transformed, its shape remains a Gaussian beam.

Among the various modes of laser beams generated by lasers, the most fundamental and widely used is the fundamental Gaussian beam mode. In a cylindrical coordinate system with the z-axis as the axis of symmetry for beam propagation, the electric field vector vibration of the fundamental Gaussian beam can be expressed as

E0​ is a constant, and the meanings of the other symbols are as follows:

In the formula:

• $w_0$ is the beam waist radius of the fundamental Gaussian beam.
• $f$ is called the confocal parameter or Rayleigh length of the Gaussian beam.
• $R(z)$ is the curvature radius of the wavefront of the fundamental Gaussian beam at the point where it intersects the propagation axis at $z$.
• $w(z)$ is the beam radius of the Gaussian beam at the point where it intersects the propagation axis at $z$.

 

Depending on the understanding method of the divergence angle, there are various measurement methods, such as:Lens method, Light intensity distribution measurement ,Double slit method.

3.4 Beam Parameter Product (BPP)

The Beam Parameter Product (BPP) is another parameter commonly used to characterize laser beam quality. According to ISO 11146 standards, BPP is given by the product of the beam radius (measured at the beam waist) and the half-angle beam divergence (measured in the far field). BPP is typically expressed in mm-mrad and can be related to the M2 factor:

The M2 factor is proportional to the BPP, so a larger BPP indicates poorer beam quality and less similarity to an ideal Gaussian beam. The minimum achievable BPP value is λ/π, corresponding to an ideal Gaussian beam. For example, the minimum possible BPP for a 1064 nm wavelength beam is approximately 0.339 mm-mrad.

BPP is commonly used for multimode semiconductor or fiber lasers with large M2 factors. Typical BPP values for high-power commercial products range from 3 to 10 mm-mrad.

 

3.5 Single-Mode and Multi-Mode Definitions

Single-Mode and Multi-Mode: When we refer to single-mode or multi-mode lasers, we are usually talking about longitudinal modes, whereas transverse modes are classified as fundamental and higher-order modes. If we relate the laser wavelength to longitudinal modes as described earlier, the distinction between single-mode and multi-mode becomes clear: single-mode lasers output light at only one wavelength, while multi-mode lasers output light that includes multiple wavelengths.

Firstly, it's important to distinguish between transverse modes and longitudinal modes because they are fundamentally different:

1. Longitudinal Modes: Longitudinal modes refer to the stable standing wave patterns formed within the laser resonator. Within a single resonator, there can be many standing wave patterns because multiple patterns can satisfy the phase matching conditions inside the cavity (assuming a wide gain spectrum in the laser’s active region). Each standing wave pattern corresponds to a longitudinal mode. To understand longitudinal modes, you can associate each mode with a specific wavelength of the laser. Essentially, each wavelength corresponds to a different longitudinal mode of the laser.

2. Transverse Modes: Transverse modes refer to the distribution of the laser field (including both electric and magnetic fields) in the cross-section perpendicular to the direction of laser propagation. Simply put, it describes how the field looks on this cross-section. For example, the fundamental mode has a single spot (or beam profile) on the cross-section, while higher-order modes have multiple spots. Note that seeing a single spot on the cross-section (fundamental mode) does not necessarily mean the laser is single-mode in the longitudinal sense.

Longitudinal modes correspond to the standing wave patterns inside the laser resonator, such as

The image shows two standing waves inside the laser resonator that satisfy the threshold gain conditions. Both standing waves can produce stable laser output, but they correspond to different wavelengths; that is, standing wave 1 corresponds to one longitudinal mode, and standing wave 2 corresponds to another longitudinal mode. If, due to various reasons, only one stable standing wave pattern remains in the laser resonator, the laser can achieve single-longitudinal-mode output. However, in the presence of certain reflectivity at the laser's end faces, a series of Fabry-Perot (FP) modes will exist inside the laser. If the power difference between the chosen main mode and the secondary modes is significant (typically, a side-mode suppression ratio greater than 30 dB is considered single-longitudinal-mode operation), the laser can be regarded as having single-longitudinal-mode output.

The figure above shows some spectral diagrams of a semiconductor laser with AR (10%) - HR (90%) simulated using MATLAB. In the first image, the laser output has a side-mode suppression ratio greater than 40 dB, indicating it can be considered single-longitudinal-mode output. In the second image, there are clearly two closely spaced longitudinal modes, suggesting it is a double-longitudinal-mode laser. Both images clearly show that due to the non-zero reflectivity of the laser end faces, it forms a Fabry-Perot (FP) cavity, resulting in a series of FP modes. However, due to the non-flat gain spectrum, some modes are extinguished during mode competition.

The three images below show the light field distribution on the cross-section of a ridge-waveguide laser. In the first image, there is a single elliptical spot, known as the fundamental mode. In the subsequent two images, the spot is divided into two and three parts, referred to as the first-order mode and second-order mode (higher-order modes), respectively. Additionally, terms such as guided modes and leakage modes are used from the perspective of transverse modes.

 

 

 

4. Laser Processing Systems

4.1 Focusing Lenses

A focusing lens is used to converge a parallel beam of light into a point. The image below shows a single-piece focusing lens. For specialized applications, there are also special types of focusing lenses. Single-piece lenses can exhibit spherical and chromatic aberrations, so special lenses are used to correct these issues, such as those designed to eliminate spherical aberration or chromatic aberration.

Spherical Aberration Correcting Focusing Lenses:

There are two types of lenses used to correct spherical aberration: compound spherical aberration-correcting lenses and aspheric lenses.

What is Spherical Aberration?

In conventional single-piece focusing lenses, which are polished with a spherical surface, spherical aberration occurs because the focal points of light rays near the center of the lens and those near the edges are not the same. This results in the light beam focusing over a length rather than a single point, leading to less concentrated energy and poorer cutting performance.

To address this issue, compound lenses made of two or three lens elements can be used to correct spherical aberration, as can aspheric lenses. Aspheric lenses are generally superior but are more expensive.

In the past, during the YAG cutting machine era, compound spherical aberration-correcting lenses and aspheric lenses were more commonly used. However, with the transition to fiber lasers, the use of these lenses has become less common.

A single positive lens produces negative spherical aberration and cannot correct spherical aberration on its own. Similarly, a single negative lens produces positive spherical aberration and cannot correct it by itself. However, when a positive lens and a negative lens are combined, and the positive lens's negative spherical aberration exactly equals the negative lens's positive spherical aberration, spherical aberration can be corrected. This is the principle behind compound spherical aberration-correcting lenses.

 

Here’s an example: Core Diameter: 100 µm ,Collimation Distance: 150mm, Focusing Distance: 250 mm

 

 

 

4.5 Practical Applications of Collimators—Classification of Collimated Focusing Head Structures

According to the structure of the collimated focusing head, there are four types: the first type is a pure collimated focusing head, which does not include any additional components like CCDs; the next three types include CCDs for trajectory calibration or in-weld monitoring, which are more common. The choice of structure is then designed based on the physical interference in different application scenarios. So, in summary, apart from special structures, most are of the third type, paired with CCDs, and do not have a special impact on the welding process; the main consideration is the on-site mechanical structure interference. Also, there are differences in straight blow heads, generally depending on the application scenario, and some may involve simulations of the protective gas flow to ensure effective protection, leading to special designs to maintain this effect.

Collimated focusing heads can be further classified into high-power and medium-to-low-power welding heads based on the application scenario. The main differences lie in the lens material and coating, which affect phenomena such as thermal drift (high-temperature focal point shift) and power loss. Thermal drift in a high-quality collimated focusing head is generally controlled within 1 mm, while lower-quality heads may exceed 2 mm. Power loss refers to the loss of power as the laser passes through the QBH head into the welding head and exits through the protective lens, with most energy being converted into heat in the lens. Typically, power loss should be within 3%, with some achieving 1% and others exceeding 5%. Therefore, these two factors are critical indicators for collimated focusing heads. It is advisable to measure these parameters yourself before use or request a relevant report from the manufacturer to ensure the product meets industrial site production requirements.

 

 4.6 Classification of Collimated Focusing Heads—Functional Introduction

The dot-ring welding head is a relatively new type of welding head that can transform a high-power laser beam from a circular spot into various forms such as ring-shaped or dot-ring shapes through beam shaping. This balances the energy distribution and is somewhat similar to turning a high-power laser into a ring-shaped spot, but with differences. Compared to a ring-shaped beam, a dot-ring head has less central energy and limited penetration capability. However, this simple approach of using a dot-ring head to achieve a laser energy distribution similar to a ring-shaped spot can provide a low-cost solution for reducing spatter, particularly advantageous in steel stitch welding due to its larger spot size and more uniform energy density. In welding high-reflective materials (such as aluminum and copper), it may be prone to weak welds.

 

4.7 How to Choose a Collimated Focusing Head:

The working distance is defined as the distance from the front mechanical edge of the lens to the focal plane or scanning plane of the objective lens. Please note not to confuse this with the Effective Focal Length (EFL) of the objective lens. EFL is measured from the principal plane to the focal plane of the optical system. The principal plane is a theoretical plane where the entire lens system can be assumed to refract.

When choosing a collimated focusing head, the primary considerations include the processing area, installation height, and the numerical aperture of the galvanometer (which is mainly related to the divergence angle of the laser—if the divergence angle is too large, it is not suitable for use with a galvanometer).

 

 

4.8 Summary of Collimating Lenses

For lenses used in laser transmission systems, the materials can be divided into two categories: transmissive materials and reflective materials. Both collimating focusing lenses and protective lenses need to use transmissive materials, which must meet the following requirements: good transmittance in the working wavelength range, high operating temperature, and low thermal expansion coefficient. Typically, fused silica is chosen as the material for collimating focusing lenses. Protective lenses are made of reflective materials, commonly K9 glass.

Reflective optical elements are created by coating a thin layer of high-reflectivity metal material onto the polished surface of glass or metal. Since reflection does not involve dispersion, the only optical characteristic of reflective materials is their reflectance for different colors of light. The coating materials for optical lenses should have the following properties:

1. Stable reflectance;
2. High thermal conductivity;
3. High melting point;

This ensures that even if the coating layer becomes dirty and absorbs excess heat, it will not crack or burn.

 

5. Structure and Principle of Galvanometer Scanners

5.1 Introduction to Laser Galvanometer Scanners

A laser scanner, also known as a laser galvanometer, consists of an X-Y optical scanning head, an electronic drive amplifier, and optical mirrors. The signals provided by the computer controller drive the optical scanning head through an amplifying circuit, thereby controlling the deflection of the laser beam in the X-Y plane.

In simple terms, a galvanometer is a scanning device used in the laser industry, professionally known as a high-speed scanning galvanometer (Galvo scanning system). The concept of a galvanometer is similar to that of a traditional ammeter, with mirrors replacing the needle and the signals from the probe being controlled by a computer using a DC signal ranging from -5V to +5V or -10V to +10V to perform the desired actions. Like the rotating mirror scanning system, this typical control system uses a pair of fold mirrors. However, instead of being driven by stepper motors, the mirrors are driven by servomotors. The use of position sensors and a negative feedback loop design further ensures system accuracy, significantly improving the system's scanning speed and repeatable positioning accuracy.

A galvanometer scanning marking head mainly consists of XY scanning mirrors, a field lens, the galvanometer itself, and computer-controlled marking software. Optical components are selected based on the laser wavelength. Related options may also include a beam expander and the laser source.

In laser display systems, the optical scanning waveform is a type of vector scan. The scanning speed of the system determines the stability of the laser graphics. In recent years, high-speed scanners have been developed, capable of reaching scanning speeds of 45,000 points per second, allowing for the demonstration of complex laser animations.

 

5.2 Laser Galvanometer Welding Heads

5.2.1 Definition and Composition of Galvanometer Welding Heads:

A collimated focusing head uses mechanical devices as a platform, with the movement of the mechanical device back and forth to achieve welding of different seam trajectories. The welding precision depends on the accuracy of the actuating mechanism, leading to issues such as low precision, slow response speed, and high inertia. In contrast, the galvanometer scanning system uses a motor to deflect mirrors, with the motor driven by a certain current, offering advantages such as high precision, low inertia, and fast response. When the laser beam hits the mirror of the galvanometer, the deflection of the galvanometer changes the angle at which the laser beam is reflected. As a result, the galvanometer system allows the laser beam to scan along any trajectory within the scanning field.

 

The main components of a galvanometer scanning system include a beam expander collimator, focusing lens, XY two-axis scanning galvanometer, control board, and upper computer software system. The scanning galvanometer primarily refers to the XY two-axis galvanometer scanning heads, which are driven by high-speed reciprocating servomotors. The dual-axis servo system sends command signals to the X and Y axis servomotors, driving the XY dual-axis scanning galvanometers to deflect along the X and Y axes, respectively. Through the combined movement of the XY two-axis mirrors, the control system can, based on the preset graphic template in the upper computer software, convert signals via the galvanometer control board and quickly move to create a scanning trajectory on the workpiece plane following the set path.

 

5.2.2 Classification of Galvanometer Welding Heads:

1. Pre-Focus Scanning Lens

   The scanning method of a galvanometer can be divided into pre-focus scanning (as shown in Figure 1 below) and post-focus scanning (as shown in Figure 2 below) based on the positional relationship between the focusing lens and the laser galvanometer. In pre-focus scanning, the laser beam experiences an optical path difference (varying beam travel distances) when deflected to different positions, resulting in a hemispherical curved focal plane during the scanning process, as illustrated on the left.

   In post-focus scanning, as shown on the right, the objective lens is an F-𝜃 flat-field lens. The F-𝜃 flat-field lens has a specialized optical design that, through the introduction of optical correction, can adjust the hemispherical focal plane of the laser beam to a flat plane. Post-focus scanning is mainly suitable for applications requiring high processing precision and a relatively small processing area, such as laser marking and laser microstructure welding.

 

2.Post-Focus Scanning Lens

 

As the scanning area increases, the aperture of the F-theta lens also needs to increase. However, due to technical and material limitations, large-aperture F-theta lenses are extremely expensive, making this option less viable. A more feasible solution is the combination of an objective-lens-front galvanometer scanning system with a six-axis robot. This approach reduces the dependence on the galvanometer equipment while maintaining a considerable degree of system precision and offering good compatibility. This method, often referred to as "flying welding," has been widely adopted by most integrators. Flying welding is used in the welding of module busbars, as well as in pole cleaning, and it allows for flexible and efficient enhancement of the processing area.

 

3. 3D Galvanometer:

Neither pre-focus nor post-focus scanning methods can dynamically control the focus of the laser beam. In pre-focus scanning, when the workpiece to be processed is small, the focusing lens has a certain depth of focus range, allowing for focused scanning over a limited area. However, when the scanning plane is large, points near the periphery may become defocused, unable to maintain focus on the surface of the workpiece, as they exceed the laser's focal depth range. Therefore, when it is necessary for the laser beam to focus well at any position within the scanning plane, and the field of view is large, a fixed focal length lens may not meet the scanning requirements.

A dynamic focusing system is an optical system with a focal length that can vary as needed. Researchers have proposed the use of dynamic focusing lenses to compensate for optical path differences. By moving a concave lens (beam expander) linearly along the optical axis, the focus position is controlled, achieving dynamic compensation for optical path differences at different positions on the workpiece surface. Compared to a 2D galvanometer, a 3D galvanometer adds a "Z-axis optical system," allowing the focus position to be freely changed during the welding process, enabling spatial curve welding without the need to adjust the welding focus position by altering the height of the carrier, such as a machine tool or robot, as required in 2D galvanometers.

 

 

  

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